In the present work, the basic features of a new reactor type, the so-called Fixed Bed Nuclear Reactor (FBNR) is presented. FBNR is a small reactor (40 MWe) without the need of on-site refueling. It utilizes the PWR technology but uses the HTGR type fuel elements. It has the characteristics of being simple in design, modular, inherent safety, passive cooling, proliferation resistant, and reduced environmental impact. The study comprises reactor description, fuel element description, criticality calculations. A series of one dimensional criticality calculations are conducted with SCALES using S-N methods. S-N calculations with SCALES have resulted for the cold reactor (20 C, 1 bar) k(infinity) = 1.4408 and for the hot reactor (308 degrees C, 160 bar), based on the average inlet-outlet temperatures, k(infinity) = 1.40003 for the spherical fuel element cell. Time calculations have been pursued for 12 years. Three dimensional criticality calculations benchmarking are conducted with MCNP5-1.4 using Monte Carlo methods and have yielded k(infinity) = 1.45673 for the hot FBNR unit cell. (C) 2007 Elsevier Ltd. All rights reserved.